Pipe-fixers include wood in an afterthought (8)

I believe the answer is:

plumbers

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'pipe-fixers include' is the definition.
I can't tell whether this defines the answer.

'wood in an afterthought' is the wordplay.
'wood' becomes 'lumber' (I've seen this before).
'in' is an insertion indicator.
'an afterthought' becomes 'ps' (ps stands for postscript).
'lumber' placed into 'ps' is 'PLUMBERS'.

Can you help me to learn more?

(Other definitions for plumbers that I've seen before include "Water or gas installations their business" , "Pipe fitters" .)

I've seen this clue in the Sydney Morning Herald.

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